1. Field of the Invention
The present invention pertains to an apparatus and method useful for selective and blanket (non-selective) epitaxial growth of silicon-containing films, where UV radiation is used to enable epitaxial film growth at temperatures of less than about 700° C., while still providing an economically feasible film growth rate.
2. Brief Description of the Background Art
Epitaxial growth of silicon-containing films has become increasingly important due to new applications for advanced logic and DRAM devices. A key requirement for these applications is a lower temperature process, so that device features will not be harmed during fabrication of the device. The lower temperature process is also important for future markets where the feature sizes are in the range of 45 nm to 65 nm, and avoidance of the diffusion of adjacent materials becomes critical. Lower process temperatures are required for both substrate cleaning prior to growth of the silicon-containing epitaxial film and during selective or blanket growth of the epitaxial film. By selective growth, it is generally meant that the silicon-containing film grows on a substrate which includes more than one material on the substrate surface; and, the silicon-containing film selectively grows on a surface a first material of said substrate, with minimal to no growth on a surface of a second material of said substrate.
Selective and blanket, non-selectively, grown silicon-containing epitaxial films, and strained embodiments of such films, including films such as Si, SixGe1-x, and SixGe1-x (C) which are grown at temperatures of less than about 700° C. are required for many current applications. Further, it would be desirable to have the removal of native oxide and hydrocarbons prior to formation of the epitaxial film accomplished at temperatures in the range of about 650° C. or less, although higher temperatures can be tolerated when the cleaning time period is short enough. Not only is the lower temperature processing important to providing a properly functioning device, but it helps prevent the relaxation of metastable strain layers, helps prevent or minimize dopant diffusion, and helps prevent segregation of dopant or Ge/C within the epitaxial film structure. Suppression of facet formation and short channel effects, which is enabled by low temperature processing (low thermal budget processing) is crucial for obtaining high performance devices. FIG. 10 shows a graph 1000 (Arrhenius plot) of the conditions required to produce an oxide-free silicon surface of the kind which is required for good epitaxial growth of silicon-containing films deposited on the surface. The inverse temperature of the substrate is shown on axis 102 in ° K−1. The corresponding temperature in ° C. is shown on axis 104 in ° C. The partial pressure of H2O present in the process chamber is shown on axis 1006 in Torr. It becomes readily apparent that to obtain an oxide-free silicon surface at 10−6 Torr, for example, it is necessary to raise the substrate temperature to about 800° C. or higher. To be able to obtain an oxide-free silicon surface at about 650° C., would require an H2O partial pressure of less than 10−8 Torr, which is not practical in terms of equipment costs.
Current techniques for selective and blanket epitaxial growth of doped and undoped Si, SiGe, and SiGe (C) films are typically carried out using reduced pressure CVD (also referred to as RPCVD or LPCVD). The typical reduced pressure process is carried out at temperatures above 700° C., typically above 750° C.) to get an acceptable film growth rate when the precursor compounds for film deposition are SiH4, SiH2Cl2 (DCS), Si2H6, GeH4. For selective deposition processes, these precursor-compounds are combined with additional reagents such as C12, HCl, and optionally HBr, by way of example. A carbon-containing silane precursor compound such as CH3SiH3 may be used as a dopant. In the alternative, inorganic compounds such as B2H6, AsH3, and PH3, by way of example, may also be used as dopants.
A plot of blanket SixGe1-x epi film growth (non-selective growth) from dichlorosilane (DCS) and germane (GeH4) is illustrated in FIG. 1. The growth rate of the SixGe1-x epi film in angstroms/minute is shown as a function of the deposition temperature and as a function of the ratio of the DCS:GeH4 in the precursor gas feed to the deposition process. This plot is for deposition of films containing from about 20% to about 28% germanium. To reduce the variation in germanium content in the deposited SixGe1-x epi film at different deposition temperatures, it was necessary to adjust the ratio of Si:Ge ratio in the precursor gases used for deposition of the film. FIG. 1 shows a graph 100 of the growth rate in Å/min of the SixGe1-x epi film on axis 104 as a function of the DCS:GeH4 precursor gas flow ratio on axis 108, and as a function of temperature in ° C. on axis 102. The data shown in FIG. 1 was generated using a process chamber of the kind described subsequently herein, using a flow rate for the DCS of approximately 100 sccm, and the flow rate for the GeH4 which was varied in the manner indicated. The film formation process illustrated in FIG. 1 was carried out at a process chamber pressure of about 10 Torr. FIG. 1 illustrates the problem of reduced SixGe1-x epi film growth rate at temperatures below about 700° C., where the growth rate decreases rapidly from about 400 Å/min at 750° C. to about 15 Å/min at 600° C. This severe decrease in film growth rate at temperatures below about 700° C. is attributed to the fact that the film growth is thermally activated and surface reaction limited.
A similar rapid decrease in film growth rate is observed for the selective deposition of silicon-germanium films. The rapid decrease in film growth rate as temperature decreases is possibly due to a high activation energy required to obtain desorption of SiCl2 from the growing film surface, a low recombinative HCl desorption rate, and low surface mobility.
It is generally known in the art that a silicon-containing substrate may be cleaned to remove organic materials and to remove native oxide from the substrate surface. In some instances such cleaning has incorporated the use of UV cleaning in a reduced pressure CVD system. Such a cleaning may be carried out at a temperature in the range of about 800° C., making use of irradiation of the substrate by deep UV radiation under hydrogen at a pressure ranging from 0.002 to 10 Torr. An apparatus of the kind used to carry out the substrate cleaning process may be a horizontal cold-wall air-cooled quartz chamber where the backside of the wafer being cleaned is heated directly by air-cooled tungsten lamps. Risetime to 900° C. and cooldown time to 200° C. are typically about 5 seconds and about 20 seconds, respectively. The base pressure in the process chamber is typically in the range of about 2 mTorr. After reaching that pressure, hydrogen flow is established. The hydrogen flow is generally for purposes of avoiding contamination of the substrate surface during the cleaning process. The cleaning of the sample is carried out at 800° C., with the sample surface under hydrogen, where the sample surface is submitted for less than one minute to short UV radiation, where the radiation is centered mostly at about 253 nm. Subsequently, an epitaxially-grown silicon (for example) film is deposited from reactive gases such as electronic grade SiH4 and HCl diluted in hydrogen (around 1/1000). Typically the substrate temperature during deposition of the epitaxially-grown silicon film is about 800° C. and higher. When the processing temperatures for film deposition were above 800° C., initial interface defects are generally avoided.
In some instances, the substrate cleaning is carried out at higher temperatures at atmospheric pressure. There are examples in which, prior to the epitaxial silicon film growth, the wafer substrate is raised to a temperature in the range of 900° C. to 1190° C., where the wafer is subjected to a 60 second hydrogen bake, followed by an etch with HCl doped hydrogen gas for a period of 30 seconds to eliminate semiconductor oxides from the wafer surface. After wafer etching, the chamber is purged with hydrogen, followed by continuing HCl gas flow and the introduction of SiHCl3, where epi growth is subsequently conducted at a temperature in the range of 1,130° C., to provide a silicon film growth rate of about 5 μ/min (50,000 Å/min).
The epitaxial growth of silicon has been carried out using rapid thermal processing techniques to reduce convective heat loss during film growth. The reactant gases used for rapid thermal processing typically include a chlorinated silicon source (e.g., SixHyClz) mixed with hydrogen, as the reactive carrier. An inert carrier gas may be added, such as argon, neon, xenon, or krypton. The process is carried out at temperatures of the silicon substrate above 750° C. under atmospheric or reduced pressure. When a dichlorosilane gas is used as a precursor, this gas is said to decompose when heated at the boundary layer on the substrate surface to SiCl2 and H2. The SiCl2 is said to diffuse to the surface, to then react with H2 to form HCl and silicon. The HCl is said to be desorbed through the boundary layer into the chamber, where it is carried away. A hydrogen carrier gas is said to be added to improve the deposition rate by supplying sufficient hydrogen concentration to fuel the decomposition of SiCl2 to Si and HCl. A pretreatment of the silicon substrate typically includes a baking step with 10% hydrogen in argon as an ambient environment. The pre-bake temperature is 1,050° C. for 5 seconds, followed by 1,000° C. for 20 seconds. The bake pressure was not specified in the literature. The silicon epi deposition was carried out at 1,000° C., at a process pressure of 50 Torr, and with a reaction mixture of argon/hydrogen gas, 10% hydrogen by volume at 18 slm, with dichlorosilane flow at 90 sccm.
The catalyst-assisted growth of a monocrystalline silicon layer on a silicon substrate during formation of a semiconductor device has also been described in the art. The single crystal silicon layer is formed on a seeding layer made having a lattice match with a single crystal silicon. The seeding layer is formed by CAD (Catalyst assisted deposition) during a CVD (Chemical Vapor Deposition) process. A seeding material such as crystalline sapphire may also be used.
In forming a single-crystal silicon layer by CAD using a catalyst, it is said to be preferable that a gas containing mainly silicon hydride is decomposed through contact with a catalyst body heated to 800-2,000° C., for example 1600-1800° C., to deposit a single-crystal silicon layer on the substrate. Silicon hydride refers to a silane, such as a monosilane, disilane, or trisilane, for example. The catalyst body is at least one kind of material selected from the group consisting of tungsten, tungsten containing thorium oxide, molybdenum, platinum, palladium, silicon, alumina ceramic with metal attached thereto, and silicon carbide. The catalyst material is typically formed as coil, above and facing the substrate. The catalyst body is a resistance wire which is activated and heated to a temperature below its melting point. The incoming silicon hydride gas, and hydrogen, or a doping gas such as B2H6 or PH3 included as necessary, are introduced to come in contact with the catalyst. By using a catalytic hot filament over the substrate surface and flowing film-forming precursor gases over that hot filament prior to deposition on the substrate, it is said to be possible to deposit an expitaxial silicon seed layer on a substrate which is at a temperature of about 100° C.-700° C., typically 200° C.-600° C. However, there is a problem with metal contamination of the film, with the contamination coming from the catalyst. Without the use of a catalyst, the silicon film formed over such processing temperatures is an amorphous silicon film.
There has recently been some description of the deposition of silicon-containing films over mixed substrates using chemical vapor deposition methods. There is description of the deposition of Si and SiGe films; however, no data is provided for the precise crystalline composition of the films which are produced by this method. The processes employ trisilane (H3SiSiH2SiH3) to enable the deposition of “high quality Si-containing films” over the mixed substrates.
In a recent publication, a chemical vapor deposition process is described which is said to make use of chemical precursors which permit deposition of thin films at or near the mass transport limited regime. The embodiments described pertain to the formation of Si-containing and Ge-containing films. The use of higher-order precursors, such as trisilane or trisilane in combination with digermane, is recommended for replacement of silane precursor, for example.
There are also references in the literature which describe pre-treatment of a silicon substrate surface prior to deposition of a silicon epitaxially-grown film on the substrate. In a process referred to as advance integrated chemical vapor deposition (AICVD) for semiconductor manufacture, the substrate is said to be prebaked at a temperature ranging from 800° C. to 900° C. for 10 to 30 minutes with hydrogen flowing in a UHV-LPCVD process chamber, to remove the native oxide from the silicon substrate surface. This is followed by growth of a medium temperature silicon epitaxial layer at a temperature in the range from 600° C. to 900° C. (typically 700° C.-800° C.) to a thickness in the range from 100 Å to 300 Å, using dichlorosilane as the source gas. Subsequent to the silicon epi growth, the dichlorosilane is replaced by silane gas or other hydrogen-containing gas which is fed to the process chamber and the temperature is dropped below 400° C., whereby the surface of the semiconductor substrate is said to be hydrogen terminated.
In another method described for the removal of a native oxide layer, hydrogen gas is used as a processing reagent. Laser projection onto the surface of the substrate is used to enhance the reaction of the hydrogen. No particular wavelength for the irradiation is mentioned. Evidently, the projected laser provides for localized heating at the substrate surface.
A review of the above-mentioned background art and other art known in the field of semiconductor manufacturing indicates that there is a continuing need for a method which enables both blanket and selective deposition of a silicon-containing, epitaxially grown films at a temperature below 750° C., and preferably below about 700° C. In addition, there is a need for a substrate cleaning procedure (in preparation for deposition of the silicon-containing film) which can be carried out at the same or a lower temperature than that of the silicon-containing film deposition, so that devices in the substrate onto which the film is being deposited will not be affected by cleaning of the substrate and deposition of the film.